This invention relates to a transmission-type liquid crystal display element which is formed by injecting a liquid crystal between a pair of transparent plates and displays picture information by the electrooptic effect of this liquid, and to a liquid crystal display using this transmission-type liquid crystal element as a light valve.
The twisted-nematic (TN)-type liquid display element as a typical example of the liquid crystal display element has a liquid crystal cell formed of a liquid crystal injected between a pair of transparent plates with transparent electrodes, and two polarizers which are disposed before and after this liquid crystal cell so that their polarizing directions are 90xc2x0 different. The amount of transmitted light of the incident light is controlled to display picture information by the combination of the polarizing plane rotating action of the electrooptical effect of the liquid crystal and the polarizing component selecting action of the polarizers. The liquid crystal display element is described in detail in, for example, xe2x80x9cFundamentals and Application of Liquid Crystal Electronicsxe2x80x9d edited by Sasaki and published by a Japanese company, Ohm Company (1979). There are known further three prior arts xe2x80x9cIntegrated Planar Micro Lens and its Applicationsxe2x80x9d Proc. Soc. Photo. Opt. Instrum. Eng., 898, 3-11 (1988); U.S. Pat. No. 5,052,783; and U.S. Pat. No. 5,056,912. This liquid crystal display element includes metal wiring for electrodes of each picture element, nonlinear elements or switching elements added as means for controlling the individual picture elements, and portions (light shielding portions) not contributing to the display such as the gaps around the electrodes of each picture element. Thus, particularly in this transmission-type liquid crystal display element, the light arrived at the light shielding portions as a part of the light which is emitted from a light source and irradiated on the liquid crystal display element is not transmitted through the liquid crystal display element, and therefore reduces the light-utilization efficiency. This light-utilization efficiency is normally expressed by the aperture ratio of the liquid crystal display element. It is important to increase this aperture ratio. The aperture ratio is defined as follows.
Aperture ratio=(Effective area contributing to the display at one picture element)/(Area of all region of one picture element)
In addition, when the liquid crystal display element is small-sized for a compact display using the liquid crystal display element, the area of one picture element is reduced more as the liquid crystal display element is small-sized, provided that the number of picture elements of the liquid crystal display element is constant. Thus, the effect of the light-shielding portions becomes great, and it is difficult to increase the light intensity. Moreover, in order to increase the definition of the liquid crystal display element with the same size and thereby to increase the resolution of the liquid crystal display using the liquid crystal display element, it is necessary to reduce the pitch of the picture elements. In that case, if all the constituents of the liquid crystal display element can be reduced analogically, the effect of the light-shielding portions are not changed, and the aperture ratio is not changed. However, the width of the metal wiring of electrodes and size of additional devices cannot be reduced to a certain value or below from the viewpoint of etching precision and alignment precision. As a result, the aperture ratio is decreased with the increase of the definition.
Examples of a transmission-type liquid crystal display element improved in the aperture ratio and liquid crystal display using this liquid crystal display element are described in, for example, Japanese Patent Laid-open Gazettes No. 60-165622 and No. 61-11788 in which the liquid crystal display element has a micro-lens array provided.
In either one of the above conventional examples, each unit lens portion of the micro-lens array is a circular lens or a semi-circular cylindrical lenticular lens. Each picture element of the above liquid crystal display element is normally of a square shape or rectangular shape. Therefore, when the micro-lens array (circular) is provided on the liquid crystal display element (square or rectangular), the light incident on the aperture area of each unit lens portion of the micro-lens array can be converged on the aperture area of the liquid crystal display element, but the light incident on the other area is incident to the light-shielding portions. Thus, all the incident light cannot effectively enter each unit lens portion, and as a result the improvement of the aperture ratio is limited.
Moreover, the liquid crystal display element has generally a large dependency on the visual angle. The liquid crystal display element can exhibit good characteristics for the viewers when it is looked up at an angle relative to the direction perpendicular to the display screen. Thus, when the liquid crystal display element is disposed to be tilted relative to the incident light, the incident light to the micro-lens array of the conventional liquid crystal display element is not converged just at the aperture area of the liquid crystal display element.
Also, in the display using the liquid crystal display element with the micro-lens array and an illumination optical system or in the projection-type display using a screen on which the image on the liquid crystal display element is projected through a projection lens, the aperture ratio improvement effect of the liquid crystal display element with the micro-lens array is changed by the illumination optical system. However, this is not considered in the above conventional liquid crystal display element and display using the same.
Accordingly, it is an object of the invention to solve the problems in the prior art and provide a liquid crystal display element capable of effectively using all the light incident to the liquid crystal display element, thus greatly improving the aperture ratio.
It is another object of the invention to provide a bright, compact and good-performance liquid crystal display using a liquid crystal display element and an illumination optical system which has a high light utilization efficiency for the light from a light source, a small number of parts such as lenses, is compact, and can further improve the effect of the liquid crystal display element of which the aperture ratio is greatly improved.
According to this invention, there is provided, in order to solve the above problem, a liquid crystal display having a light source 1 for emitting light, an illumination optical system for irradiating the light from the light source 1 onto a liquid crystal display element 4 in which an optical image according to a video signal is formed, and said liquid crystal display element 4 as a transmission-type liquid crystal display element which has a pair of transparent plates 11 and a liquid crystal injected therebetween and displays picture information by the electrooptical effect of the liquid crystal, wherein a flat micro-lens array 16 having an array of unit lenses respectively opposing to the picture elements of the picture element array of the liquid crystal display element and having the same array as the picture element array of the liquid crystal is provided to be in intimate contact with the surface of the liquid crystal display element to which the light from the light source is incident or to both surfaces thereof including the surface from which the incident light exits or to be integral with the transparent plates 11, and the focal points of these unit lenses are formed at around the liquid crystal surface of the liquid crystal display element and substantially at the center of theand electrodes of the picture elements. In addition, the flat micro-lens array 16 is formed as a refractive-index distribution-type flat plate micro-lens array in which regions of a refractive index N different from the refractive index N0 are periodically formed within the substrate. Also, the micro-lens array 16, or the refractive-index distribution-type flat plate micro-lens array is formed by overdiffusion so that the shapes of the regions acting as the unit lenses are, respectively, equal or substantially equal to the picture elements of the liquid crystal display element 4.
In the liquid crystal display using the liquid crystal display element according to this invention, the illumination optical system including the light source is constructed as follows. The illumination optical system has the light source 1 for emitting the light, and a concave mirror 2 and condenser lens group 3 for irradiating the light from the light source 1 onto the light valve 4 on which an optical image according to a video signal is formed. The concave mirror 2 is formed elliptical, and the light source 1 is provided at around the elliptical focal point close to the center of the elliptical concave mirror 2a. The condenser lens group 3 or a part thereof is provided at around the elliptical focal point of the concave mirror 2a on the side facing the liquid crystal display element 4. Alternatively, the concave mirror 2 is formed parabolic, and the condenser lens group 3 having a positive refractive power or a part thereof is provided at around the side of the concave mirror 2b from which the light from the light source 1 exits.
Moreover, the illumination optical system is constructed so that the light source 1 and the concave mirror 2 are formed approximately similar to the shape of the aperture of each picture element of the liquid crystal display element 4 through which light is transmitted, in the cross-section perpendicular to the optical axis of the illumination optical system.
According to the liquid crystal display element of the invention constructed as mentioned above, the incident light is effectively conducted to the picture element electrodes, and is almost not xe2x80x9ceclipsedxe2x80x9d by the metal wiring of electrodes, nonlinear devices or switching devices added as means for controlling the picture elements individually, and the gaps around the picture element electrodes (light-shielding portions) not contributing to the display. Thus, bright picture information can be displayed even if the aperture ratio is poor (small). In addition, according to this invention, since the light exiting from the liquid crystal display element is only the effective light for the display, the opaque portions between the picture elements are eliminated, and thus picture information can be smoothly displayed.
Also, according to the illumination optical system of the above construction, since the light from the light source is once focused by the refractive power of the condenser lens group properly located, and light at around the focal point can be controlled, it is not necessary to increase the aperture of the condenser lens, and the incident angle of light to the liquid crystal display element or the micro-lens array relative to the optical axis can be decreased to improve the focusing characteristic of the micro-lens array, so that the light utilization efficiency can be increased. Furthermore, since the shape of the luminous flux passing through the apertures in the cross-section perpendicular to the optical axis, or the shape of the light source as viewed from the micro-lens, or the shape of the light source in the cross-section perpendicular to the optical axis, is approximately similar to the shape of the apertures, the luminous flux passing through the apertures of the liquid crystal display element covers all the range of the apertures, so that the liquid crystal display has a large aperture ratio, or is able to display bright and clear picture information.